Because the 3 answers consumed 3 ids, it is impossible for a question to have ID #2, #3, or #4. This is the behavior as defined in the question linked to above.

However, this is not reflected in the API. In some cases, you must know the type of a post in order to request information about it. For example, if you want to request the comments of a particular post, you must use either /questions/{id}/comments or /answers/{id}/comments. It would be really wonderful if I could just use /posts/{id}/comments and not have to rely on knowing the post type in order to retrieve its comments.

@Kevin it makes library implementation slightly simpler. Specific example: retrieve the comments to a particular post. Instead of having to know that the post is a Q or an A, I could just request /posts/{id}/comments and be done with it. (which I realize now isn't immediately obvious in my question. i'll update it)
–
Dave DeLongMay 29 '10 at 20:50

1

There's some merit to a posts/{id}/comments method. However, if any method might return different types (for instance, a hypothetical /posts/{id}) I'd have to say its not worth complexity.
–
Kevin Montrose♦May 30 '10 at 5:18